Чего-то не понимаю, что не так с последним условием?
package com.javarush.task.task21.task2104;
import java.util.HashSet;
import java.util.Set;
/*
Equals and HashCode
*/
public class Solution {
private final String first, last;
public Solution(String first, String last) {
this.first = first;
this.last = last;
}
public boolean equals(Object n) {
if (n==this){return true;}
if (!(n instanceof Solution)){return false;}
Solution volz = (Solution) n;
if(volz.first!=this.first){return false;}
if(volz.last!=this.last){return false;}
boolean res=false;
if (volz.first==null&this.first==null&volz.last==this.last){res=true;}
if (volz.last==null&this.last==null&volz.first==this.first){res=true;}
return res;
}
public int hashCode() {
int res= this.first!=null?31*this.first.hashCode():0;
return this.last!=null?res+31*this.last.hashCode():res;
}
public static void main(String[] args) {
Set<Solution> s = new HashSet<>();
s.add(new Solution("Donald", "Duck"));
System.out.println(s.contains(new Solution("Donald", "Duck")));
System.out.println(s.contains(new Solution(null, null)));
System.out.println(s.contains(new Solution(null, "Duck")));
System.out.println(s.contains(new Solution("Donald", null)));
}
}